1. Field of the Invention
This invention relates in general to an apparatus used in the fabrication of a semiconductor device, and more particularly to an apparatus to aid in the maintenance of the chemical-mechanical-polishing (CMP) process for planarizing a material layer in a semiconductor device.
2. Description of Related Art
It is essential in the fabrication of semiconductor wafers to make the surfaces of the wafers flat for photolithography. Thus, during this fabrication process, it is necessary to polish the wafer to selectively remove excess material from its surface. In a chemical-mechanical-polishing (CMP) process, semiconductor substrates are rotated against a polishing pad covered by a layer of polishing slurry. CMP is used for planarizing bare silicon wafers, interlevel dielectrics, and other materials.
CMP involves both chemical and mechanical abrasion. Chemical abrasion is accomplished through a slurry to chemically weaken the surface of the wafer. Mechanical abrasion is accomplished using a polishing pad against which a wafer surface is pressed.
The slurry chemically degrades the wafer surface to make the surface more easily removed by the mechanical abrasion. One example of a slurry usable for polishing deposited layers on semiconductor wafers is a colloidal suspension of silica particles in an approximately 10.5 pH solution of water and KOH.
The material of the polishing pad is chosen for its ability to act as a carrier of the slurry and to wipe away the grit and debris resulting from the polishing action.
Chemical compounds within the wafer slurry undergo a chemical reaction with material of the insulating layer to enhance the rate of removal. However, debris on the surface of the polishing pad can be formed by the accumulation of chemical reaction products and abrasives in the slurry, which may reduce the polishing rate because the mass transfer rate of the polishing slurry is reduced. This effect is called "glazing". Thus, after continued use, the polishing surface of the pad will deteriorate and need replacement.
Limited prior art techniques have introduced a number of methods to reduce the glazing effects. In one method, ultrasonic energy was introduced in the polishing slurry to overcome the instability caused by glazing of the polishing pad. In another, a brush was used on a polishing cloth to remove debris.
In U.S. Pat. No. 5,522,965, issued to Chisholm, et al., on Jun. 4, 1996, entitled, "COMPACT SYSTEM AND METHOD FOR CHEMICAL-MECHANICAL POLISHING UTILIZING ENERGY COUPLED TO THE POLISHING PAD/WATER INTERFACE", the approach to conditioning consists of a transducer emitting ultrasonic energy during the polishing process. However, unlike the present invention, there is no cleaning step, abrasive conditioning, or vacuuming of the pad surface.
Similarly, in U.S. Pat. No. 5,384,986, issued to Hirose, et al., on Jan. 31, 1995, entitled, "POLISHING APPARATUS", polishing of the silicon wafer is detailed, but not pad maintenance. In Hirose, et al., an abrasive cloth is used for polishing, not a conventional polyurethane pad. A rotatable brush oscillating between inner radial and outer radial positions on the abrasive cloth (in lieu of a pad) is used to remove debris and reduce glazing. Although a cleaning solution is sprayed over this cloth material, no abrasive conditioning of the cloth is described.
Spray bars have also been developed to clean the polishing pad. In U.S. Pat. No. 5,578,529, issued to Mullins, on Nov. 26, 1996, entitled, "METHOD FOR USING RINSE SPRAY BAR IN CHEMICAL MECHANICAL POLISHING", a rinse bar was added to CMP equipment to provide uniform wetting and rinsing of the polishing pad. Similarly, in U.S. Pat. No. 5,154,021, issued to Bombardier, et al., on Oct. 13, 1992, entitled, "PNEUMATIC PAD CONDITIONER", an air jet assembly supported over a polishing pad provided for raising flattened fibers on the pad pressed down by the polishing of semiconductor wafers, and for blowing spent polishing materials and by-products off the pad. However, these patents do not cover any abrasive conditioning or vacuuming of the pad surface.
The prior art has limitations in the implementation of the CMP process. Polishing actions must be carried out in such a way that scratches or other defects do not appear on the polished wafer surface. Additionally, in order to achieve uniform planarity a constant polishing rate must be maintained when using a pad that requires conditioning. Pad conditioning would be desirable to reduce glazing effects, otherwise the pad's polishing rate and operating life are appreciably degraded.
Consequently, it has been found that active pad maintenance is critical to reducing the disadvantages associated with the CMP process.
Bearing in mind the problems and deficiencies of the prior art, it is therefore an object of the present invention to provide an apparatus for maintaining a polishing pad used in a chemical/mechanical polishing (CMP) process.
It is another object of the present invention to provide a pad maintenance method to efficiently condition the pad and clear debris.
It is another object of the present invention to provide for the removal of particles and debris on the polishing pad surface while effectively conditioning the pad surface during a CMP process.
It is another object of the present invention to minimize the scratching of product wafers during the CMP process.
It is another object of the present invention to maintain a constant polishing rate when using a pad that requires conditioning.
It is yet another object of the present invention to extend the polishing pad's life through the minimization of accumulated debris on the pad's surface.
Still other objects and advantages of the invention will in part be obvious and will in part be apparent from the specification.